NOx emissions from vehicles with internal combustion engines are an environmental problem recognized worldwide. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations. In conventional gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel powered vehicles and vehicles with lean-burn gasoline engines, however, the exhaust is too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel powered vehicles and lean-burn gasoline engines. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and homogenizing fuel-air mixtures can reduce NOx emissions. These techniques alone, however, will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOx catalysts, NOx adsorber-catalysts, and selective catalytic reduction (SCR).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proved challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. Lean-burn NOx catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean burn NOx catalysts typically employ a zeolite wash coat, which is thought to provide a reducing microenvironment. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiency with lean-burn catalysts is unacceptably low.
NOx adsorber-catalysts alternately adsorb NOx and catalytically reduce it. The adsorber can be taken offline during regeneration and a reducing atmosphere provided. The adsorbant is generally an alkaline earth oxide adsorbant, such as BaCO3 and the catalyst can be a precious metal, such as Ru. A drawback of this system is that the precious metal catalysts and the adsorbant may be poisoned by sulfur.
SCR involves using ammonia as the reductant. The NOx can be temporarily stored in an adsorbant or ammonia can be fed continuously into the exhaust. SCR can achieve NOx reductions in excess of 90%. One concern relates to controlling the ammonia feed rate. The NOx flow rate and demand for ammonia vary widely and rapidly during engine operation. Too little ammonia can lead to NOx breakthrough and too much ammonia can result in ammonia release, which is an environmental hazard.
U.S. Pat. No. 4,963,332 describes a control scheme for SCR reduction of NOx in flue gases where the NOx concentration and mass flow rate are measured upstream of the reactor and NOx concentration is also measured downstream of the reactor. The mole ratio of ammonia feed to NOx is adjusted based on the downstream NOx concentration. U.S. Pat. No. 4,751,054 describes a similar approach using an ammonia sensor.
U.S. Pat. No. 5,522,218 describes a control scheme for NOx reduction in diesel exhaust where the reductant is supplied according to a feed forward control scheme based on engine operating conditions and exhaust gas temperature. The reductant supply rate is determined by a table look-up.
U.S. Pat. No. 5,047,220 describes a feed-forward control scheme to establish a supply rate of reductant at 90% of estimated requirements and a feed-back loop to set a trim signal establishing a supply rate for the balance of the required reductant.
U.S. Pat. No. 4,314,345 describes a feed forward control scheme for NOx reduction in flue gases in which the ammonia supply rate is adjusted during exhaust gas temperature transients to account for temperature-dependent increases and decreases in the amount of ammonia adsorbed in the SCR reactor.
U.S. Pat. No. 5,833,932 describes an SCR reactor for treating diesel exhaust, the reactor having a reductant storage capacity that increases along the reactor's length in the direction of flow. The low capacity up front is said to enhance light-off performance. The large capacity downstream is intended to provide a buffer against sudden increases in demand. It is also said that during transients that involve a sudden temperature increase, reductant desorbed at the front of the reactor can be captured near the back.
U.S. Pat. No. 5,785,937 describes a feed-forward control system for supplying an SCR reactor in a diesel exhaust system. The reducing agent is sometimes fed super-stoichiometrically and sometimes fed sub-stoichiometrically during transients with the objective of maintaining an optimal level of adsorbed ammonia in the SCR reactor.
U.S. Pat. No. 5,643,536 describes a feed-back control system for supplying ammonia to an SCR reactor in a diesel exhaust system wherein the control system is said to measure the thickness of a reaction zone. The thickness of the reaction zone is the depth within a porous wall of the catalyst at which the ammonia concentration passes through a minimum. The feed rate of ammonia is adjusted to seek a targeted reaction zone thickness.
U.S. Pat. No. 5,628,186 describes a feed-forward control system for supplying ammonia to an SCR reactor in a diesel exhaust system wherein the feed rate is adjusted to account for the rate of adsorption or desorption of reductant from the catalyst bed.
After reviewing many of the above-cited references, U.S. Pat. No. 6,662,553 concludes “there are no commercially available NOx sensors which have the response time needed for vehicular applications” and that “any SCR control system for mobile applications will necessarily be open loop.”
U.S. Pat. No. 6,455,009 describes a feedback control system for supplying ammonia to an SCR reactor wherein feedback is provided by a sensor cross-sensitive to ammonia and NOx. The feed rate of ammonia is continuously cycled. When the detection signal is found to be increasing while ammonia feed rate is also increasing, the feed rate is switched to a decreasing trend, optionally following a step decease. When the signal again begins to rise, the feed rate trend is again reversed.
U.S. Pat. No. 6,625,975 describes a system for supplying ammonia to an SCR reactor in a diesel exhaust system wherein a sensor that is cross-sensitive to oxidizable species, but not NOx, is used to measure ammonia concentration for feed-back control. Oxidizable species other than ammonia are removed prior to the SCR reactor by an oxidative catalytic converter.
While a great deal of effort has already been expended in this area, there continues to be a long felt need for reliable, affordable, and effective systems for controlling ammonia supply rates to SCR reactors in diesel exhaust systems.